Leila de Lima, a prominent human rights advocate and former secretary of justice, told the Guardian that deadly anti-drug operations under Duterte could amount to crimes against humanity and should be investigated by the International Criminal Court in the Hague.

“It [I.C.C.] should start to think about investigating already or doing an inquiry into the killings as crimes against humanity,” she told the newspaper in an interview.

De Lima, who has emerged as one of the president’s sole domestic critics, said Duterte’s public exhortations for alleged drug users and dealers to be killed had inspired lawlessness and vigilantism.

“You cannot dismiss this sort of rhetoric as just a figure of speech or hyperbole because that’s not how the people on the ground have been interpreting the pronouncements of the president,” she said.

The senator has faced extreme and abusive fallout from her vocal opposition to the president’s tactics. She received numerous death threats after an unknown opponent shared her personal contact information online, and her critics have accused her of corruption and sexual misconduct.

De Lima has dismissed the allegations as an attempt at character assassination.

Since Duterte took office in June, more than 3,600 people have been killed in the name of his merciless war on drugs. More than half are believed to have died at the hands of vigilantes in extrajudicial killings that targeted both drug dealers and users.

The bloody drug war has been condemned by many foreign powers including the country’s long-time ally, the United States, and the United Nations, but the criticism has been met with vitriolic rhetoric from Duterte, who is notorious for his deeply offensive, ranting insults directed at Western leaders.